1. Field of the Invention
The present invention relates to a semiconductor device provided on an insulating surface of a substrate.
2. Description of the Prior Art
In recent years, liquid crystal electro-optical devices, specifically liquid crystal display devices, have been substituted for CRT, due to demands for thin and light devices. Particularly, liquid crystal displays in which nematic liquid crystal, ferroelectric liquid crystal, polymer dispersed liquid crystal, or the like is driven by the use of thin film transistors (simply referred to as TFT hereinafter) of active matrix type have been developed and utilized in the field of large-sized projection TVs, large-sized plane TVs, and color displays for personal computers. The active matrix TFT utilized for a liquid crystal display is manufactured by providing electric leads, electrodes, an insulating layer and the like on a glass substrate by sputtering, vacuum deposition method, CVD, and the like. An example of structure of a conventional active matrix device is illustrated in FIG. 6(A). This active matrix device is an inverse stagger type insulated gate field effect transistor where a gate electrode 37 is provided on a substrate 40 and further an insulating layer 38, a semiconductor layer 36, and electrodes 34, 39 are provided thereon.
In such an active matrix TFT, a gate lead 35 connected to the gate electrode 37 and a source lead 41 connected to the source electrode 34 are necessarily crossed over the substrate. In order to prevent the gate lead 35 and source lead 41 from being electrically connected to each other at the solid crossings, an insulating film is inserted between the gate lead 35 and the source lead 41 in general. However, depending on the structure and quality of the inserted insulating film, current leakage might occur between the gate lead 35 and the source lead 41 at the solid crossings. For this reason, a thick insulating film has to be provided to prevent the leakage.
However, the more the thickness of the insulating film is increased, the larger the difference in height between a lower lead and an upper lead becomes. Accordingly, disconnection of the upper lead (i.e. source lead 41 in the case of FIG. 6(B)) is probable, as known as a step coverage problem in the semiconductor field. Further, when such a thin film transistor is utilized in a liquid crystal display device, the height of the solid crossings and that of a display electrode 39 becomes different. That means, when two substrates are mated to compose a liquid crystal cell, the thickness of the cell becomes ununiform. This results in color shade and degradation of display. From the above reason, a technique to thin solid crossings, i.e. a technique to make the thickness of an active matrix type liquid crystal cell uniform, has been required.